FIG. 9 is a sectional view depicting a structure of capacitor 90 used for a capacitor device of the prior art. Capacitor element 30 includes aluminum foils, each having a polarizable electrode layer formed thereon, which serve as electrodes (i.e., collectors). A pair of the electrodes are arranged in positions shifted with respect to each other, with a separator placed between them, and this pair of the electrodes and the separator are wound together. Capacitor element 30 made in this manner is provided with a positive electrode and a negative electrode tapped individually from both end surfaces (i.e., the top and the bottom sides in the drawing of FIG. 9).
Capacitor 90 comprises metal case 31 made of aluminum encasing capacitor element 30 together with an activating electrolyte, although not show in the figure, and negative electrode terminal 31a formed integrally on the bottom of metal case 31 for external connection. The end surface at the negative electrode side of capacitor element 30 is connected both mechanically and electrically to the inner base of metal case 31 by such means as laser welding.
Capacitor 90 further comprises cover 32 made of aluminum, and positive electrode terminal 32a formed integrally with cover 32 for external connection. The end surface at the positive electrode side of capacitor element 30 is connected to cover 32 by such means as the laser welding. This also ensures the mechanical and electrical connection. At the same time, a circumferential fringe of cover 32 and an opening edge of metal case 31 are crimped together with an interlayer of an insulating material placed between them, though not shown in the figure, to form crimped portion 33. The capacitor is sealed in this manner by the process of crimping.
A capacitor device of the prior art contains a plurality of capacitors, each having positive electrode terminal 32a and negative electrode terminal 31a for external connections along a longitudinal axis in the center of metal case 31 (i.e., a vertical orientation in FIG. 9), which are coupled by using a connecting member called a bus bar although not shown in FIG. 9. The capacitor device containing the plurality of coupled capacitors 90 is used as a backup power supply or the like of a vehicle.
One example of the above prior art is disclosed in Japanese Patent Unexamined Publication, No. 2000-315632.
In the capacitor device of the prior art, however, it has not been so easy to make connections of positive electrode terminals 32a and negative electrode terminals 31a in the process of coupling the plurality of capacitors 90 since these terminals are extended out to the opposite sides with respect to each other. This device has also had a problem that it cannot be reduced in size because it requires connecting spaces at both sides, which inevitably results in an overall large mounting space.
Moreover, when the above conventional capacitor device having a plurality of coupled capacitors 90 is used as an auxiliary power supply for a motor of a fuel cell vehicle, it necessitates lengthy routing of connecting lines for the reason that the terminals are located at both ends opposite to each other. Consequently, there has been a problem that a large loss occurs due to an undesirable increase in resistance.
Description is now provided about this undesirable increase in resistance by using an example of a fuel-cell stack for a fuel cell vehicle, with reference to FIGS. 10A and 10B.
Fuel-cell stack 51 of the fuel cell vehicle supplies a voltage of 400V, as shown in FIG. 10A. On the other hand, capacitors 60 of capacitance “Ci” have a rated voltage of approx. 2.0V. It is therefore necessary to provide 200 units of capacitors (i.e., 400V/2V=200) connected in series. A load current of approx. 200 A flows at the maximum. Here, load “LO” includes a motor load and a motor controller load.
Each of the capacitors has an internal resistance “R1” of 0.002Ω, as shown in FIG. 10B. Assuming that there is connection resistance “R2” of 0.0001Ω at terminals of the individual capacitors, it adds up to 0.0021Ω (i.e., 0.002+0.0001=0.0021) when connected in series. A total resistance thus becomes 0.002Ω×200=0.42Ω in the case of the 200 units of serially connected capacitors.
When the current of 200 A flows through the combined resistance of 0.42Ω, it generates 16.8 kW of heat (i.e., 0.42Ω×200 A×200 A=16.8 kW) as a loss of the capacitors, which is dissipated outside.
In the case of the conventional capacitor device, there has been the problem as described above, that a substantial amount of heat is dissipated due to the large loss generated by the resistance attributed to the connections.